One who truly believes that Jesus Christ is Lord and savior, the way the truth and the light, the only way to the father, believes he was fully god and fully man who came to earth Messiah to fulfill scriptures and die in exchange for our sins so that we could get access to the father even as sinners. characteristics that are observable by man are generally a changed life that they live for his glory. This can be acted out a number of ways but feeding his sheep and spreading the gospel are the most common. Agape!
I would also be wary of using Baptism as a way to tell if someone is Christian or not. Mark 16:16 “Whoever believes and is baptized will be saved, but whoever does not believe will be condemned.”
Notice the word believes? It’s faith. If you truly believe, your actions will follow. If you get baptized without belief ( and this verse indicates it’s possible), you can lie all you want, but you can’t play games with the Lord.we are saved by Grace through faith and Baptism is something we then do out of joy and obedience to the Lord and as a public proclamation of that faith to the world. To look strictly at the act of baptism as a way to determine someone’s faith is simply checking a box to get Christian street
Creds. Pharisees did this too. Be wary of using this as your measure, friend. Agape!